Process for production of hydrocarbons

ABSTRACT

A process for converting synthesis gas composed of hydrogen and carbon monoxide to hydrocarbons. The process includes the step of contacting at reaction conditions a synthesis gas feed to a catalyst which includes cobalt in catalytically active amounts up to about 60 wt % of the catalyst and rhenium in catalytically active amounts of about 0.5 to 50 wt % of the cobalt content of the catalyst supported on alumina. A metal oxide promoter may be added.

This application is a continuation-in-part of Ser. No. 113,095, filedOct. 23, 1987, now U.S. Pat. No. 4,801,573.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for converting synthesis gasto hydrocarbons, and more particularly to a process using a catalystcomprising cobalt and rhenium on an alumina support.

2. Description of the Prior Art

The reaction to convert carbon monoxide and hydrogen mixtures (definedherein as synthesis gas or syngas) to higher hydrocarbons over metalliccatalysts has been known since the turn of the century. This reaction iscommonly referred to as the Fischer-Tropsch or F-T synthesis. DuringWorld War II, Germany exploited a process employing the F-T synthesisfor the production of gasoline and other hydrocarbon products. By 1944 atotal of nine F-T plants was operating in Germany. The German processused primarily a catalyst composed of cobalt, magnesium oxide, thoriumoxide and kieselguhr, in the relative proportions of 100:5:8:200. Later,most of the thoria was replaced by magnesia, primarily for economicreasons. Currently, commercial Fischer-Tropsch plants are operating inSouth Africa. These plants use a process employing a precipitatediron-based catalyst which contains various promoters to improve thestability and product distribution.

The common F-T catalysts are nickel, cobalt and iron. Nickel wasprobably the first substance to be recognized as capable of catalyzingthe reaction of syngas to hydrocarbons, producing mainly methane (see,for example, "The Fischer-Tropsch Synthesis" by R. B. Anderson, AcademicPress (1984), p. 2). Iron and cobalt are able to produce longer chainlength hydrocarbons and are thus preferred as catalysts for theproduction of liquid hydrocarbons. However, other metals are alsocapable of catalyzing the F-T synthesis. Ruthenium is a very activecatalyst for the formation of hydrocarbons from syngas. Its activity atlow temperatures is higher than that of iron, cobalt or nickel; and itproduces a high proportion of heavy hydrocarbons. At high pressures, itproduces a high proportion of high molecular weight wax. Osmium has beenfound to be moderately active, while platinum, palladium and iridiumexhibit low activities (see Pichler, "Advances in Catalysis", vol. IV,Academic Press, N.Y., 1952). Other metals which are active, such asrhodium, yield high percentages of oxygenated materials (Ichikawa,Chemtech, 6, 74 (1982)). Other metals that have been investigatedinclude rhenium, molybdenum and chromium, but these exhibit very lowactivities with most of the product being methane.

Various combinations of metals can also be used for hydrocarbonsynthesis. Doping cobalt catalysts with nickel causes an increase inmethane production during F-T synthesis (see "Catalysis", vol. IV,Reinhold Publishing Co., (1956), p. 29). In U.S. Pat. No. 4,088,671 toT. P. Kobylinski, entitled "Conversion of Synthesis Gas Using aCobalt-Ruthenium Catalyst", the addition of small amounts of rutheniumto cobalt is shown to result in an active F-T synthesis catalyst with alow selectivity to methane. Thus, these references teach that thecombination of two or more metals can result in an active F-T catalyst.In general, the catalysts of these teachings have activities andselectivities which are within the ranges of the individual components.

Combinations of metals with certain oxide supports have also beenreported to result in an improved hydrocarbon yield during F-Tsynthesis, probably due to an increase in the surface area of the activemetal. The use of titania to support cobalt or cobalt-thoria is taughtin U.S. Pat. No. 4,595,703, entitled "Hydrocarbons from Synthesis Gas".In this case the support serves to increase the activity of the metal(s)toward hydrocarbon formation. In fact, titania belongs to a class ofmetal oxides known to exhibit strong metal-support interactions and, assuch, has been reported to give improved F-T activity for a number ofmetals(see, for example, S. J. Tauster et al, Science, 211, 1121(1981)). Combinations of titania and two or more metals have also beenshown to yield improved F-T activity. In U.S. Pat. No. 4,568,663,entitled "Cobalt Catalysts In the Conversion of Methanol to Hydrocarbonsand for Fischer-Tropsch Synthesis", combinations of cobalt, rhenium andthoria and cobalt and rhenium supported on titania are claimed usefulfor the production of hydrocarbons from methanol or synthesis gas. Thispatent also indicates that similar improvements in activity can beobtained when cobalt-rhenium or cobalt-rhenium-thoria is compounded withother inorganic oxides. However, titania is the only supportspecifically discussed. The typical improvement in activity gained bypromotion of cobalt metal supported on titania with rhenium is less thana factor of 2. We have found that the addition of rhenium to cobaltmetal supported on a number of other common supports results in similarimprovements in activity.

The only other examples in the literature of catalysts involvingmixtures of cobalt and rhenium refer to completely different chemicalreactions. For example, in Soviet Union Pat. No. 610558, a catalystcomposed of cobalt and rhenium supported on alumina is taught to resultin improved performance for the steam reforming of hydrocarbons. Steamreforming of hydrocarbons is a process completely different fromhydrocarbon production via F-T synthesis and is believed to proceed by acompletely different mechanism. Although some steam reforming catalystscan convert synthesis gas to hydrocarbons, such catalysts are notselective for the production of high carbon-number hydrocarbons (C₃ andabove) during conversion of synthesis gas. In fact, most commonly usedsteam reforming catalysts contain nickel as their active metal, andnickel produces mostly methane when used for syngas conversion.

SUMMARY OF THE INVENTION

It has been found in accordance with the present invention thatsynthesis gas comprising hydrogen and carbon monoxide can be convertedto liquid hydrocarbons by using a process employing a catalystconsisting of cobalt and rhenium supported on alumina. The processincludes the step of contacting a synthesis gas feed comprises ofhydrogen and carbon monoxide over a catalyst comprised of cobalt andrhenium composited on an alumina support wherein rhenium is present inrelatively lesser amounts than the cobalt content of the catalyst. Thereaction conditions include a temperature in the range of about 150° to325° C., a pressure in the range of about atmospheric to 100 atmospheresand a gaseous hourly space velocity, based on the total amount ofsynthesis gas feed, in the range of about 0.1 to 50 standard m³ of gasper kilogram of catalyst per hour. (Standard conditions are defined as apressure of 1 atmosphere and a temperature of 60° F.) The catalyst usedin the process of this invention preferably contains from about 5 to 60%cobalt and has a rhenium content between 0.5 and 50% of the amount ofcobalt. The alumina preferably is gamma alumina.

It has been found that the addition of small amounts of rhenium tocatalysts consisting predominantly of cobalt supported on aluminaunexpectedly results in greatly enhanced activity of this catalyst forhydrocarbon production from syngas. This is surprising in light of thefact that rhenium supported on alumina shows very low activity, withmost of the product being methane. Furthermore, rhenium addition tocobalt supported on supports other than alumina results in catalystswith much lower activity levels. In addition, the more active cobaltplus rhenium catalyst maintains the high selectivity to higherhydrocarbons and the low selectivity to methane found with analumina-supported cobalt catalyst. Both the high activity and the lowmethane production of cobalt-rhenium on alumina are unexpected in lightof the facts that (1) rhenium shows very low activity for F-T synthesis,(2) the main products from F-T synthesis over a rhenium catalyst aremethane and carbon dioxide, and (3) the use of alumina as a support forcatalysts containing only cobalt results in no, or at least only aslight, increase in activity compared to the use of cobalt on othersupports. Thus, for reasons not fully understood, the combination ofcobalt and rhenium supported on alumina results in a catalyst which issignificantly more active than either of the two individual metalssupported on alumina or the combination of the two metals supported onother inorganic supports, such as silica, magnesia, silica-alumina,titania, chromia or zirconia. Furthermore, the product distribution witha high selectivity to C₂ + hydrocarbons and low selectivity to methaneand carbon dioxide would not have been predicted based on the knownproduct distribution from rhenium catalysts.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a graph showing the effect of rhenium content on CO conversionusing catalysts containing 12% cobalt;

FIG. 2 is a graph showing the effect on CO conversion of adding rheniumto catalysts containing various amounts of cobalt on an alumina support;

FIG. 3 is a typical graph of CO conversion as a function of time onstream for the process of this invention when operated using a slurryreactor; and

FIG. 4 is a gas chromatagram of a typical middle distillate and heavierliquid product from the process of this invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The process of the present invention comprises contacting at reactionconditions a feed comprised of hydrogen and carbon monoxide over acatalyst which comprises as the active catalytic ingredients cobalt andrhenium supported on alumina with rhenium present in a relativelysmaller amount than cobalt.

Operating conditions suitable for use in the process of this inventionare a reaction temperature between 150° and 325° C., preferably between180° and 280° C., and more preferably between 190° and 250° C.; a totalpressure from about 1 atmosphere to around 100 atmospheres, preferablybetween 1 and 40 atmospheres, and more preferably between 1 and 30atmospheres; and a gaseous hourly space velocity, based on the totalamount of synthesis gas feed, between 0.1 and 50 standard m³ of gas perkilogram of catalyst per hour, and preferably from 0.1 to 20 standard m³/kg/h, where gaseous hourly space velocity is defined as the volume ofsynthesis gas (measured at standard temperature and pressure) fed perunit weight of catalyst per hour.

The catalyst used in the process of this invention has been found to behighly active for the conversion of synthesis gas, a mixture of hydrogenand carbon monoxide, into a mixture of predominantly paraffinichydrocarbons. As indicated above, it has long been known that cobalt isan active catalyst for the F-T synthesis. It is also known that theaddition of rhenium to a cobalt catalyst supported on titania givesimproved activity, even if rhenium by itself shows very low activity forF-T synthesis and produces methane as the main product. Surprisingly, wehave found that the choice of support for the cobalt plus rheniumcatalyst is very critical, and that the addition of rhenium to analumina-supported cobalt catalyst gives a much higher improvement inactivity than addition of rhenium to cobalt supported on other inorganicoxides.

The cobalt is added to the alumina support in some amount up to about 60wt% of the catalyst, including cobalt. Preferably, amounts between 5 and45 wt% are used; and more preferably between 10 and 45 wt%. The contentof rhenium is between about 0.5 and 50 wt% of the cobalt content;preferably between 1 and 30 wt%; and more preferably from about 2 toaround 20 wt%.

In addition to cobalt and rhenium, it is beneficial to include a smallamount of a metal oxide promoter in an amount between about 0.1 and 5wt%, and more preferably between about 0.2 and 2 wt%, based on theweight of the complete catalyst. The promoter is suitably chosen fromelements in groups IIIB, IVB or VB of the periodic chart, thelanthanides and the actinides. The promoter oxide can be chosen from,for example, Sc₂ O₃, Y₂ O₃, La₂ O₃, Ce₂ O₃, Pr₂ O₃, ZrO₂, Ac₂ O₃, PaO₂,Nd₂ O₃, CeO₂, V₂ O₅ or Nb₂ O₅. The most preferable oxide is La₂ O₃, or amixture of lanthanides, rich in lanthanum. Oxides like MnO or MgO canalso be included. While not essential, the use of these metal oxides iscommon in the art, since they are believed to promote the production ofproducts with higher boiling points, while maintaining or improvingcatalytic activity. However, the catalyst is highly active and selectivewithout the addition of a promoter.

THE CATALYST SUPPORT

The catalytically active metals and the promoter metal oxide, ifpresent, are distended on alumina. Although other supports may be used,it has been found, for example, that silica, titania, chromia, magnesia,silica-alumina and zirconia produce catalysts with much loweractivities.

To be most effective when used as a support, alumina should becharacterized by low acidity, high surface area, and high purity. Theseproperties are necessary in order to enable the catalyst to have highactivity and a low deactivation rate, and to produce high molecularweight hydrocarbon products. The surface area of the alumina support isat least, and preferably greater than, about 100 m² /g; and morepreferably greater than 150 m² /g. The pore volume is at least, andpreferably greater than, about than 0.3 cm³ /g. The catalyst supportmust be of high purity. That is, the content of elements, e.g. sulfurand phosphorous, that have a deleterious effect on catalytic activitymust be kept low. The sulfur content of the catalyst support should bekept below 100 ppm and preferably below 50 ppm. Although gamma aluminahas generally been used and is preferred, a number of aluminastructures, if prepared properly, can meet these conditions and aresuitable supports. For example, eta-alumina, xi-alumina, theta-alumina,delta-alumina, kappa-alumina, boehmite and pseudo-boehmite can all beused as supports.

CATALYST PREPARATION

The method of depositing the active metals and the promoter oxide on thealumina support is not critical, and can be chosen from various methodswell known to those skilled in the art. One suitable method that hasbeen employed is known as incipient wetness impregnation. In this methodthe metal salts are dissolved in an amount of a suitable solvent justsufficient to fill the pores of the catalyst. In another method, themetal oxides or hydroxides are coprecipitated from an aqueous solutionby adding a precipitating agent. In still another method, the metalsalts are mixed with the wet support in a suitable blender to obtain asubstantially homogeneous mixture. In the present invention, ifincipient wetness impregnation is used, the catalytically active metalscan be deposited on the support using an aqueous or an organic solution.Suitable organic solvents include, for example, acetone, methanol,ethanol, dimethyl formamide, diethyl ether, cyclohexane, xylene andtetrahydrofuran. Aqueous impregnation is preferred when Co(NO₃)₂ is usedas the salt, while an organic solvent is the preferred solvent when thecatalyst is prepared from cobalt carbonyl.

Suitable cobalt compounds include, for example, cobalt nitrate, cobaltacetate, cobalt chloride and cobalt carbonyl, with the nitrate being themost preferable when impregnating from an aqueous solution. Suitablerhenium compounds include, for example, rhenium oxide, rhenium chlorideand perrhenic acid. Perrhenic acid is the preferred compound whenpreparing a catalyst using an aqueous solution. The promoter cansuitably be incorporated into the catalyst in the form, for example, ofthe nitrate or chloride.

After aqueous impregnation, the catalyst is dried at 110° to 120° C. for3 to 6 hours. When impregnating from organic solvents, the catalyst ispreferably first dried in a rotary evaporator apparatus at 50° to 60° C.under low pressure, then dried at 110° to 120° C. for several hourslonger.

The dried catalyst is calcined under flowing air by slowly increasingthe temperature to an upper limit of between 200° and 500° C.,preferably between 250° and 350° C. The rate of temperature increase ispreferably between 0.5° and 2° C. per minute, and the catalyst is heldat the highest temperature for a period of 2 to 5 hours. Theimpregnation procedure is repeated as many times as necessary to obtaina catalyst with the desired metals content. Cobalt, rhenium and thepromoter, if present, can be impregnated together, or in separate steps.If separate steps are used, the order of impregnating the activecomponents can be varied.

Before use, the calcined catalyst is preferably reduced with hydrogen.This can suitably be done in flowing hydrogen at atmospheric pressure ata flow rate between 30 and 100 cm³ /min when reducing about 2 g ofcatalyst. The flow rate should suitably be increased for largerquantities of catalyst. The temperature is increased at a rate between0.5° and 2° C. per minute from ambient to a maximum level of 250° to450° C., preferably between 300° and 400° C., and maintained at themaximum temperature for about 6 to 24 hours, more preferably 10 to 24hours.

After the reduction step, the catalysts may be oxidized and reducedbefore use. To carry out the oxidation step, the catalyst is treatedwith dilute oxygen (1-3% oxygen in nitrogen) at room temperature for aperiod of 1/2 to 2 hours before the temperature is increased at the samerate and to the same temperature as used during calcination. Afterholding the high temperature for 1 to 2 hours, air is slowly introduced,and the treatment is continued under air at the high temperature foranother 2 to 4 hours. The second reduction is carried out under the sameconditions as the first reduction.

HYDROCARBON SYNTHESIS

The reactor used for the synthesis of hydrocarbons from synthesis gascan be chosen from various types well known to those skilled in the art,for example, fixed bed, fluidized bed, ebullating bed or slurry. Becauseof the exothermic nature of the F-T reaction, the reactor must bedesigned with heat removal capabilities so that the desired reactiontemperature can be carefully controlled. The above listed reactor typeshave characteristics that make them well suited for use in the processof this invention. The catalyst particle size for the fixed orebullating bed is preferably between 0.1 and 10 mm and more preferablybetween 0.5 and 5 mm. For the other types of reactors a particle sizebetween 0.01 and 0.2 mm is preferred.

The synthesis gas used as feed to the process is a mixture of carbonmonoxide and hydrogen and can be obtained from any source known to thoseskilled in the art, such as, for example, steam reforming of natural gasor partial oxidation of coal. The molar ratio of H₂ :CO is preferablybetween 0.5:1 to 3:1; and preferably between 1:1 to 3:1 and morepreferably between 1.5:1 to 2.5:1. Carbon dioxide is not a desired feedcomponent for use with the process of this invention, but it does notadversely affect the process, other than acting as a diluent. All sulfurcompounds must, on the other hand, be held to very low levels in thefeed, preferably below 1 ppm, because they have an adverse effect on theactivity of the catalyst employed in this process.

The process of this invention is described as follows. Synthesis gasfrom any suitable source, as discussed previously, is fed to theprocess. The molar ratio of hydrogen to carbon monoxide in thissynthesis gas may be between 1 and 3 and more preferably between 1.5 and2.5. A highly desirable ratio is the stoichiometric ratio of H₂ to COusage within the process, which is about 2.1 to 2.2. Providing syngas atthis stoichiometric ratio results in the most efficient utilization ofthe syngas in the process, since neither hydrogen nor carbon monoxide isin excess. In addition to H₂ and CO, the syngas may contain quantitiesof other gases, such as carbon dioxide, methane, and nitrogen. Thesegases act as diluents which may be disadvantageous for some kinds ofreactor systems, while being advantageous for other reactor systems, aswill be more fully discussed below. The hydrogen sulfide content of thesyngas must be kept very low, preferable below 1 ppm by volume, becausesulfur is a severe catalyst poison.

If not at sufficient pressure, the syngas is compressed to processpressure, which can be from 1 atmosphere to about 100 atmospheres,preferably from 1 to 40 atmospheres, and more preferably from 1 to 30atmospheres. Anything lower than 1 atmosphere would require operation atvacuum conditions which is not necessary and unduly expensive. The rateof reaction would decrease. Pressures greater than about 100 atmosphereswould increase the cost significantly due to the increased strength ofthe equipment necessary to withstand high pressures. The syngas is thenpreheated before entering the reactor. Because the F-T reaction ishighly exothermic, it is not normally necessary to heat the feed gas allthe way to reaction temperature, the final heatup taking place in thereactor itself. As discussed previously, the reactor can suitably bechosen from a variety of reactor types, the most important criterionbeing the ability to control carefully the temperature of the exothermicF-T reaction. The three most suitable reactor types are tubular fixedbed reactors, in which the catalyst is placed in tubes and a fluid iscirculated on the outside of the tubes for heat removal, fluidized bedreactors, and slurry reactors, in which finely divided catalyst isslurried in a vehicle oil. In these latter two reactor types, heat canbe removed in a variety of ways, including increases in the sensibleheat of the feed, internal heat exchangers, and removal of a slip streamfor cooling and return to the reactor.

Temperature in the reactor should be between 150° and 325° C., and morepreferably between 180° and 280° C. These temperature ranges are typicalto Fischer-Tropsch reactions. The rate of reaction at temperatures below150° C. would be so low as to be uneconomical on a commercial scale dueto the large reactor size which would be required. Where temperaturesabove about 325° C. are emloyed, the selectivity to liquid hydrocarbonsis so low that the process becomes economically unfeasible, with much ofthe product being methane. There are less expensive ways to makemethane. Gaseous hourly space velocity (based only on the H₂ plus COcontent of the feed) should be between 0.1 and 50 m³ per kg catalyst perhour and preferably between 0.1 and 20 m³ per kg catalyst per hour. Oncethe desired working temperature and pressure are determined, the spacevelocity is chosen to give the desired per pass conversion.

The acceptable form of the catalyst will depend upon the type of reactorbeing used. For tubular fixed bed reactors, the catalyst can be in theform of extrudates, pellets, spheres, granules, etc. with a nominaldiameter of about 0.5 to 6 mm, and preferably about 1.5 mm. Forfluidized bed or slurry reactors, the catalyst should be in finelydivided form. A typical analysis for a catalyst suitable for slurryreactor operation is:

    ______________________________________                                        Particle Diameter, microns                                                                     Weight % of Sample                                           ______________________________________                                        0-5              0.2                                                          5-7              1.5                                                          7-9              1.3                                                          9-13             0.5                                                          13-19            1.1                                                          19-27            1.5                                                          27-38            3.3                                                          38-53            5.7                                                          53-75            15.7                                                         75-106           25.7                                                         106-150          26.4                                                         150-212          13.9                                                         212-300          3.2                                                          ______________________________________                                    

For either fluidized bed or slurry operation, it is important that thecatalyst charge contain neither too large a fraction of large particlesnor too large a fraction of small particles. A proper size consist isimportant for successful fluidization or suspension of the catalyst. Inslurry operation, it is also important to have a proper vehicle.Normally, a fraction of the product oil will be used for this purpose.Generally, a carbon number distritation in the range from C₂₀ to C₅₀ issatisfactory. For startup purposes, in addition to a F-T liquid, C₃₀ toC₅₀ poly-alpha-olefins may be used or a highly refined, i.e. heteroatomand aromatic free, petroleum oil.

In the reactor, contact between the syngas and the catalyst results inthe production of largely paraffinic hydrocarbons, along with smallamounts of olefins and oxygenates. In general, part of the product willremain in the gas phase and be carried out of the reactor along withinerts and unconverted feed gas, and part of the product will form aliquid phase. The fraction of the product that is carried out in thevapor phase will depend upon the operating conditions being used.

In a fixed bed reactor, the liquid phase will either drain out thebottom of the catalyst tubes or else be entrained out with the offgas.In a fluidized bed reactor, operating conditions must be adjusted sothat the amount of liquid product is very low. Otherwise, the productbuilds up on the catalyst and destroys the fluidization properties ofthe catalyst. In the slurry reactor, the liquid products will dissolvein the vehicle oil and can be recovered by any of the techniquesfamiliar to those skilled in the art, such as filtration,centrifugation, settling and decanting, etc.

The offgas from the reactor is cooled to condense liquid products (bothhydrocarbons and water, which is produced as a byproduct). This istypically done in a series of steps at progressively lower temperatures.This is necessary to prevent wax from solidifying and causing plugging.After the final cooler/separator, additional hydrocarbons can berecovered by absorption or adsorption, if desired.

Per pass conversion in the reactor can vary from 10 to over 90%,preferably from 40 to over 90%. If once through conversion is notsufficiently high, the offgas, after product recovery and bleeding offof a slip stream to prevent inerts from building up in the system, canbe mixed with the fresh syngas feed and recycled to the reactor.

As mentioned above, the syngas may contain some quantity of nitrogen.For fixed bed operation, this may be undersirable, since dilution withnitrogen reduces the partial pressure of reactor gases and increases thepressure drop. However, for fluidized bed and slurry reactor operation,the nitrogen may be beneficial by providing additional mixing energy andhelping keep the catalyst suspended. In these types of reactors,pressure drop is not a strong function of flow rate.

As a final step in the process, all the liquid products may be combined.If desired they may be stabilized by distillation to remove highlyvolatile components. The liquid product may then be marketed as asynthetic crude or, alternatively, distilled into individual productcuts which can be marketed separately. As a further alternative, theproduct may be catalytically dewaxed or hydrocracked before beingmarketed. These latter processes can improve product properties bylowering pour point, increasing octane number, and changing boilingranges.

The products from this process are a complicated mixture, consistingpredominantly of normal paraffins, but also containing small amounts ofbranched chain isomers, olefins, alcohols and other oxygenatedcompounds. The main reaction can be illustrated by the followingequation:

    nCO+2nH.sub.2 →(--CH.sub.2 --).sub.n +nH.sub.2 O

where (--CH₂ --)_(n) represents a straight chain hydrocarbon of carbonnumber n. Carbon number refers to the number of carbon atoms making upthe main skeleton of the molecule. Products range in carbon number fromone to 50 or higher.

In addition, with many catalysts, for example those based on iron, thewater gas shift reaction is a well known side reaction:

    CO+H.sub.2 O→H.sub.2 +CO.sub.2

With the catalyst used in the process of this invention, the rate ofthis last reaction is usually very low. However, it is found that, eventhough rhenium catalysts exhibit a relatively high selectivity to carbondioxide, the cobalt plus rhenium catalyst used in the process of thisinvention surprisingly does not have a higher selectivity to carbondioxide than the cobalt only catalyst.

The hydrocarbon products from processes employing the Fischer-Tropschsynthesis are generally distributed from methane to high boilingcompounds according to the so called Schulz-Flory distribution, wellknown to those skilled in the art. The Schulz-Flory distribution isexpressed mathematically by the Schulz-Flory equation:

    W.sub.i =(1-α).sup.2 iα.sup.i-1

where i represents carbon number, α is the Schulz-Flory distributionfactor which represents the ratio of the rate of chain propagation tothe rate of chain propagation plus the rate of chain termination, andW_(i) represents the weight fraction of product of carbon number i.

The products produced by the process of this invention generally followthe Schulz-Flory distribution, except that the yield of methane isusually higher than expected from this distribution. This indicates thatmethane is apparently produced by an additional mechanism.

It is well known, and also known in one of the following examples, thatrhenium alone is a low activity catalyst for Fischer-Tropsch synthesisproducing a product which is predominantly methane. On the other hand,cobalt is a well known catalyst for producing higher carbon numberhydrocarbons. In U.S. Pat. No. 4,568,663, it has been shown that addingsmall amounts of rhenium to cobalt supported on titania improves thecatalytic activity. In the present invention, it has been found that thehydrocarbon yield obtained by adding rhenium is surprisingly much largerfor an alumina supported cobalt catalyst than that obtained from cobaltand rhenium on several other inorganic supports. The improved activityis followed by no deleterious effect on the selectivity to methane.

The process of this invention is further described in the followingexamples.

EXPERIMENTAL WORK

The following examples describe the preparation of various catalysts andthe results obtained from testing these catalysts for conversion ofsynthesis gas into hydrocarbons.

Before being tested, each catalyst was given a pretreatment consistingof reduction by passing hydrogen over the catalyst at a rate of 3000 cm³/g/h while heating the catalyst at a rate of 1° C./min to 350° C. andmaintaining this temperature for 10 hours. In all the process testsexcept for Example 42, synthesis gas consisting of 33 vol% carbonmonoxide and 67 vol% hydrogen was passed over 0.5 g of the catalyst in asmall fixed bed reactor at atmospheric pressure at temperatures of 185°,195° and 205° C. according to the following schedule:

    ______________________________________                                                 9 hr. 50 min. at 195° C.                                               4 hr. 20 min. at 205° C.                                               4 hr. 30 min. at 185° C.                                               9 hr. 50 min. at 195° C.                                      ______________________________________                                    

The flow rate of synthesis gas was 1680 cm³ /g of catalyst/h. Productsfrom the reactor were sent to a gas chromatograph for analysis.Catalysts were compared based on the results over the period from 10 to30 hours on stream.

EXAMPLE 1 CATALYST CONTAINING COBALT BUT NO RHENIUM

This example describes the preparation of a control cobalt catalystwhich was used for comparative purposes. This catalyst was prepared asfollows:

A solution was prepared by dissolving 17.03 g of cobalt nitrate,Co(NO₃)₂.6H₂ O, and 0.76 g of mixed rare earth nitrate, RE(NO₃)₃, whereRE stands for rare earth with a composition of 66% La₂ O₃, 24% Nd₂ O₃,8.2% Pr₆ O₁₁, 0.7% CeO₂, and 1.1% other oxides (Molycorp 5247), in 30 mlof distilled water. The total solution was added with stirring to 25 gof Ketjen CK300 gamma-alumina which had been calcined 10 hours at 500°C. The prepared catalyst was then dried for 5 hours in an oven at atemperature of 115° C. The dried catalyst was then calcined in air byraising its temperature at a heating rate of 1° C./minute to 300° C. andholding at this temperature for 2 hours. The finished catalyst contained12 wt% cobalt and 1 wt% rare earth oxide with the remainder beingalumina. This catalyst is referred to as preparation "a" in Table I. Theabove procedure was repeated to produce preparation "b" catalyst inTable I.

The results of the tests with this catalyst are shown in Table I. Inthis and the following tables, selectivity is defined as the percent ofthe carbon monoxide converted that goes to the indicated product.

                  TABLE I                                                         ______________________________________                                                      CO                                                                            Con-     C.sub.2 +                                                                             CH.sub.4                                                                              CO.sub.2                               Temp. Prepa-  version  Selectivity                                                                           Selectivity                                                                           Selectivity                            °C.                                                                          ration  %        %       %       %                                      ______________________________________                                        185   a       7        91.1    7.2     1.7                                          b       11       91.8    7.1     1.1                                    195   a       12       90.0    8.9     1.1                                          b       18       90.2    9.0     0.8                                    205   a       21       87.7    11.3    1.0                                          b       29       86.7    12.4    0.9                                    ______________________________________                                    

This example shows that a cobalt catalyst exhibits good selectivity toethane and longer chain length hydrocarbons and low selectivity tomethane and carbon dioxide.

EXAMPLE 2 CATALYST CONTAINING RHENIUM BUT NO COBALT

This example describes a rhenium catalyst prepared for comparativepurposes. The procedure employed was the same as for Example 1 exceptthat the solution contained 0.33 g of perrhenic acid, HReO₄ as 82.5%aqueous solution, and 0.54 g of rare earth nitrate to make 24 ml ofsolution which then was added to 20 g of calcined alumina. The finishedcatalyst contained 1 wt% rhenium and 1 wt% rare earth oxide with theremainder being alumina.

The results of the tests with the catalyst of Example 2 are shown inTable II.

                  TABLE II                                                        ______________________________________                                               CO        C.sub.2 +  CH.sub.4                                                                              CO.sub.2                                  Temp.  conversion                                                                              Selectivity                                                                              Selectivity                                                                           Selectivity                               °C.                                                                           %         %          %       %                                         ______________________________________                                        185    0.3       20         30      50                                        195    0.3       19         31      50                                        205    0.3       19         31      50                                        ______________________________________                                    

EXAMPLE 3 CATALYST CONTAINING RHENIUM BUT NO COBALT

Repetition of the procedure from Example 2, except that 0.83 g ofperrhenic acid were used, gave a catalyst containing 4 wt% rhenium. Theresults of the tests with the catalyst of Example 3 are shown in TableIII.

                  TABLE III                                                       ______________________________________                                               CO        C.sub.2 +  CH.sub.4                                                                              CO.sub.2                                  Temp.  conversion                                                                              Selectivity                                                                              Selectivity                                                                           Selectivity                               °C.                                                                           %         %          %       %                                         ______________________________________                                        185    0.3       20         30      50                                        195    0.3       19         31      50                                        205    0.3       19         31      50                                        ______________________________________                                    

The results from Examples 2 and 3 show that catalysts containing rheniumbut no cobalt have very low activity for producing desirable liquidhydrocarbons from synthesis gas. Furthermore, about half the product iscarbon dioxide, and most of the hydrocarbon product is methane.

EXAMPLES 4 THROUGH 11 CATALYSTS CONTAINING BOTH COBALT AND RHENIUM

The preparation procedure of Example 1 was employed except that varyingamounts of perrhenic acid were added to the solution. This produced aseries of catalysts containing 12 wt% cobalt and 0.1, 0.2, 0.3, 0.5,1.0, 2.0, 4.0, and 8.0 wt% rhenium in addition to 1.0 wt% rare earthoxide.

The results of the tests with the catalysts of Examples 4 through 11 at195° C. are shown in Table IV and further illustrated in FIG. 1. FIG. 1shows the effect on carbon monoxide conversion of adding rhenium tocatalysts containing 12% cobalt.

                  TABLE IV                                                        ______________________________________                                                              CO     C.sub.2 +     CO.sub.2                           Ex-                   con-   Selec-                                                                              CH.sub.4                                                                              Selec-                             ample Co      Re      version                                                                              tivity                                                                              Selectivity                                                                           tivity                             No.   wt %    wt %    %      %     %       %                                  ______________________________________                                        4     12      0.1     26     89.8  9.6     0.6                                5     12      0.2     29     88.9  10.4    0.7                                6     12      0.3     27     88.2  11.0    0.8                                7     12      0.5     31     88.3  10.9    0.8                                8     12      1.0     33     87.7  11.4    0.9                                9     12      2.0     31     85.7  13.3    1.0                                10    12      4.0     28     84.7  14.2    1.1                                11    12      8.0     25     84.5  14.2    1.3                                ______________________________________                                    

As can be seen from comparison of the results in Table I with Table IVand FIG. 1, the addition of small amounts of rhenium to a cobaltsupported on alumina catalyst significantly increases the conversion ofthe carbon monoxide in the feed. Levels of rhenium as low as 0.1 wt%result in approximately doubling the CO conversion. The exact level ofRe for optimum activity is very important, as the rate of carbonmonoxide conversion increases rapidly at low rhenium addition levels,reaches a maximum and then decreases gradually at levels greater than 1wt% rhenium. However, even at the highest rhenium level investigated(8%), a clear improvement in conversion is evident when compared to thecatalyst not containing rhenium.

It is important that the increase in activity occur without acorresponding increase in either the methane or the carbon dioxideselectivities. Table IV shows that the increase in carbon monoxideconversion is not accompanied by any substantial change in either theselectivities to methane or carbon dioxide. Thus, after rhenium additionthe principal reaction products are still desirable hydrocarbons.

EXAMPLES 12 THROUGH 25 CATALYSTS CONTAINING BOTH COBALT AND RHENIUM

The preparation procedure if Example 1 was employed except that varyingamounts of cobalt nitrate and perrhenic acid were added to the solution.This produced a series of catalysts containing from 3.0 to 40 wt% cobaltand from 0 to 5.0 wt% rhenium in addition to 1.0 wt% rare earth oxide.

The results of the tests with the catalysts of Examples 12 through 25 at195° C. are shown in Table V.

                  TABLE V                                                         ______________________________________                                                              CO     C.sub.2 +     CO.sub.2                           Ex-                   Con-   Selec-                                                                              CH.sub.4                                                                              Selec-                             ample Co      Re      version                                                                              tivity                                                                              Selectivity                                                                           tivity                             No.   wt %    wt %    %      %     %       %                                  ______________________________________                                        12    3       0.0     5      90.7  8.1     1.2                                13    3       0.25    4      87.2  10.4    2.4                                14    6       0.0     12     90.0  8.9     1.1                                15    6       0.5     16     88.2  10.8    1.0                                16    9       0.0     15     90.0  9.1     0.9                                17    9       0.75    25     88.1  11.1    0.8                                18    20      0.0     20     89.3  9.8     0.9                                19    20      0.5     40     87.9  11.1    1.0                                20    20      1.0     46     86.1  12.9    1.0                                21    20      5.0     42     83.9  14.8    1.3                                22    40      0.0     20     89.3  9.7     1.0                                23    40      1.0     56     85.0  13.2    1.8                                24    40      2.0     58     84.3  13.7    2.0                                25    40      5.0     60     81.9  15.7    2.4                                ______________________________________                                    

The results in Table V show that for cobalt catalysts without rhenium,there is a significant increase in activity in going from 3% cobalt to6% cobalt. However, only modest increases in activity occur from thispoint up to cobalt loadings of as high as 40%. At a cobalt loading of3%, the addition of rhenium does not improve the catalytic activity, butthe improvement upon rhenium addition is significant for higher cobaltloadings. In fact, the improvement in activity due to the addition ofrhenium increases as the cobalt content increases as shown in FIG. 2.

EXAMPLES 26 AND 27 COBALT/RHENIUM CATALYSTS WITH PROMOTERS

To illustrate the use of promoters other than rare earth oxides, thefollowing catalysts were prepared. The preparation procedure used toprepare the catalyst of Example 8 was used except that zirconiumnitrate, Zr(NO₃)₄, or vanadyl oxalate, VO(C₂ O₄ H)₃, was substituted forthe rare earth nitrate. The results of tests at 195° C. with thecatalysts of Examples 26 and 27 are shown in Table VI. In addition tothe promoter, these catalysts contained 12% cobalt and 1% rhenium andwere supported on alumina.

                  TABLE VI                                                        ______________________________________                                                                     C.sub.2 +                                                                           CH.sub.4                                                                            CO.sub.2                             Ex-                CO        Selec-                                                                              Selec-                                                                              Selec-                               ample              conver-   tivity                                                                              tivity                                                                              tivity                               No.   Promoter     sion %    %     %     %                                    ______________________________________                                        26    ZrO.sub.2 (0.75 wt %)                                                                      31        87.9  11.3  0.8                                  27    V.sub.2 O.sub.5 (0.56 wt %)                                                                26        89.4  9.8   0.8                                  ______________________________________                                    

EXAMPLES 28 THROUGH 41 COBALT/RHENIUM CATALYSTS ON OTHER SUPPORTS

For comparison with alumina, several catalysts were prepared on othersupports. The preparation procedure used to prepare the catalyst ofExample 8 was repeated, but without the addition of rare earth oxide.The titanium-supported catalysts were prepared on titania calcined atboth 500° C. and 600° C. After calcination at 600° C., the titania ismainly in the crystalline rutile form; while after calcination at 500°C. the anatase:rutile ratio is about 1:1. The catalysts prepared on thetitania support calcined at these two temperatures showed exactly thesame catalytic activity.

The supports used were: Davison Grade 59 silica; Degussa P25 titania;Alpha Chemicals No. 88272 chromia; magnesia prepared by calciningFischer basic magnesium carbonate; American Cyanamid AAA Silica-Alumina;and Alpha Chemicals 11852 zirconia (containing 2% alumina). Informationon the ciomposition of the catalysts prepared on the different supportsis given in Table VII.

                                      TABLE VII                                   __________________________________________________________________________                     Weight of Materials in                                                                   Composition of                                               Weight of                                                                           Impregnating                                                                             Finished                                          Example    Support                                                                             Solution, g                                                                              Catalyst, wt %                                    No.  Support                                                                             g     Co(NO.sub.3).sub.2                                                                  HReO.sub.4 *                                                                       Co  Re                                            __________________________________________________________________________    28   Silica                                                                              20    13.47 --   12  --                                            29   Silica                                                                              20    13.62 0.38 12  1.0                                           30   Titania**                                                                           25    16.84 --   12  --                                            31   Titania**                                                                           24.64 16.78 0.46 12  1.0                                           32   Titania***                                                                          25    16.48 --   12  --                                            33   Titania***                                                                          24.64 16.78 0.46 12  1.0                                           34   Chromia                                                                             20    13.47 --   12  --                                            35   Chromia                                                                             21.3  14.51 0.40 12  1.0                                           36   Magnesia                                                                            21.59 14.54 --   12  --                                            37   Magnesia                                                                            14.54 10.67 0.29 12  1.0                                           38   Silica-                                                                       Alumina                                                                             20    13.47 --   12  --                                            39   Silica-                                                                       Alumina                                                                             20    13.62 0.38 12  1.0                                           40   Zirconia                                                                            20    13.47 --   12  --                                            41   Zirconia                                                                            20    13.62 0.38 12  1.0                                           __________________________________________________________________________     *Weight of 82.5% perrhenic acid solution.                                     **Calcined at 500° C.                                                  ***Calcined at 600° C.                                            

A series of tests was conducted to evaluate the activities of thecatalysts of the above examples in converting synthesis gas intohydrocarbons. The results of the tests with the catalysts of Examples 28through 41 at 195° C. are shown in Table VIII. The results fromcatalysts prepared on alumina are included for comparison.

                                      TABLE VIII                                  __________________________________________________________________________                   CO    C.sub.2 +                                                                          CH.sub.4                                                                           CO.sub.2                                       Example                                                                            Co                                                                              Re      Conversion                                                                          Selec-                                                                             Selec-                                                                             Selec-                                         No.  % % Support                                                                             %     tivity %                                                                           tivity %                                                                           tivity %                                       __________________________________________________________________________    1    12                                                                              --                                                                              Al.sub.2 O.sub.3                                                                    12    90.0 8.9  1.1                                            8    12                                                                              1 Al.sub.2 O.sub.3                                                                    33    87.7 11.4 0.9                                            28   12                                                                              --                                                                              SiO.sub.2                                                                           11    90.1 8.7  1.2                                            29   12                                                                              1 SiO.sub.2                                                                           12    88.1 10.7 1.2                                            30   12                                                                              --                                                                              TiO.sub.2 *                                                                         11    87.6 11.8 0.6                                            31   12                                                                              1 TiO.sub.2 *                                                                         17    86.5 12.8 0.7                                            32   12                                                                              --                                                                              TiO.sub.2 **                                                                        11    87.6 11.7 0.7                                            33   12                                                                              1 TiO.sub.2 **                                                                        17    85.8 13.5 0.7                                            34   12                                                                              --                                                                              Cr.sub.2 O.sub.3                                                                    1     83.5 15.5 1.0                                            35   12                                                                              1 Cr.sub.2 O.sub.3                                                                    2     80.8 12.3 6.9                                            36   12                                                                              --                                                                              MgO   0.3   20.0 30.0 50.0                                           37   12                                                                              1 MgO   0.3   19.1 30.9 50.0                                           38   12                                                                              --                                                                              SiO.sub.2 /Al.sub.2 O.sub.3                                                         5     76.3 22.2 1.5                                            39   12                                                                              1 SiO.sub.2 /Al.sub.2 O.sub.3                                                         6     78.6 19.8 1.6                                            40   12                                                                              --                                                                              ZrO.sub.2                                                                           4     80.9 16.3 2.8                                            41   12                                                                              1 ZrO.sub.2                                                                           7     78.8 18.7 2.5                                            __________________________________________________________________________     *Support calcined at 500° C.                                           **Support calcined at 600° C.                                     

The catalysts in Table VII were prepared to test the teaching thatvarious inorganic supports are acceptable for preparing cobalt plusrhenium F-T catalysts. An examination of the data in Table VIII leads tothe surprising conclusion that the type of support is extremelyimportant and that vast differences in activity exist between catalystsprepared on one support and catalysts of the same catalytic metalscontent on another support. More surprisingly, only cobalt plus rheniumon alumina showed a commercially attractive activity level andselectivity.

Catalysts on magnesia and chromia exhibited extremely low activities,both with and without rhenium. Catalysts on zirconia and silica-aluminashowed somewhat higher activities, but selectivity to C₂ + hydrocarbonswas poor. These catalysts showed only modest improvements in activityupon the addition of rhenium.

Catalysts without rhenium supported on silica and titania showedactivity levels close to comparable cobalt on alumina catalyst. However,upon addition of rhenium, the alumina catalyst showed a surprisingincrease in activity from about 15% carbon monoxide conversion to 33%carbon monoxide conversion; whereas, the silica supported catalystshowed only a very small increase in activity from 11% carbon monoxideconversion to 12% carbon monoxide conversion, while the titaniasupported catalyst showed a larger, but still modest, gain in activityfrom 11% carbon monoxide conversion to 17% carbon monoxide conversion.

From these example, plus those presented previously, it can be concludedthat the catalytic activity of a cobalt catalyst supported on alumina isgreatly improved by adding minor amounts of rhenium, as long as thecobalt level is greater than about 5 wt%. Although improved activityfrom rhenium addition is also observed for some other supports, theactivity level achieved by adding rhenium to a catalyst supported onalumina is much higher than for other supports. This result issurprising and would not have been predicted based on teachings in theprior art.

EXAMPLE 42

A catalyst was prepared according to the procedure of Example 24, exceptthat Harshaw 4100P alumina was used as the support. Before being testedin a slurry reactor, the catalyst was pretreated as follows. One hundredgrams of this catalyst was loaded into a vertical pipe pretreatingreactor constructed of 5 ft. of 1 in. OD schedule 40 stainless steelpipe. Hydrogen was introduced to the bottom of the pipe reactor at arate of 1900 Scc/min., which was sufficient to fluidize the catalyst inthe pretreating reactor. After a fluidized bed was established,temperature was increased at the rate of 1° C./min. to a maximumtemperature of 350° C. The catalyst was held at this temperature for 16hr. and then cooled to 50° C. At this point, the hydrogen was replacedwith He, and the catalyst was cooled to ambient temperature. 14.1 g ofthis reduced catalyst were mixed with 206 grams of Synfluid (Synfluid 8cSt PAO, Chevron Chemical Company) and loaded into a 1 in. ID by 3 ft.long slurry reactor. A mixture of CO, H₂, and N₂ in a ratio of 1:2:3 wasfed to the reactor at a rate of 1080 Sl/hr. Temperature was increased to225° C. and pressure was increased to 450 psig (approximately 31atmospheres). These conditions were maintained for a total of 388 hr.,except for two periods, one of 68 hr. and the other of 94 hr., duringwhich pure N₂ was fed to the reactor. After a transition period of 2hr., there then followed a period of 90 hr. during which the ratio ofCO:H₂ :N₂ in the feed was 1:3:4. There then followed a period of 53hours during which operation was unstable due to problems withtemperature control. During this period some high temperatures wereexperienced, followed by a short period of no activity from which thecatalyst quickly recovered. It is postulated that during the hightemperature period, an abundance of light hydrocarbons were formed whichdiluted the vehicle liquid and caused collapse of the slurry bed. Thisillustrates the importance of maintaining the proper vehicle properties,mixing energy, etc. It also illustrates the inherent ruggedness of theprocess, since it was able to recover from this incident. This wasfollowed by 160 hr. during which the ratio of CO:H₂ :N₂ in the feed was1:1:2. After a 7 hr. transition period, this was followed by a period of115 hr. in which the ratio of CO:H₂ :N₂ in the feed was again 1:2:3.After a 9 hr. transition period, there then followed a period of 157 hr.with a CO:H₂ :N₂ ratio in the feed gas of 2:3:5. Finally, after a 5 hr.transition period, there was a period of 94 hr. with a CO:H₂ :N₂ feedratio of 2:5:7. Altogether from startup to shutdown, the run lasted 1080hr.

FIG. 3 is a graph of CO conversion as a function of time on stream. Thisgraph demonstrates the stability of the process and shows that the rateof deactivation is low. During operation, product was removedcontinuously from the unit, so that the slurry volume in the reactorremained constant. FIG. 4 is a gas chromatogram of a sample of themiddle distillate and heavier liquid product. The product ispredominately normal paraffins with a typical Schulz-Flory distribution,showing that the catalyst is promoting the Fischer-Tropsch reaction.

This example demonstrates that the catalyst of this invention is activefor the Fischer-Tropsch process. Moreover, it shows that the catalystmay be used in a slurry reactor, as well as in a fixed bed reactor, asillustrated by the previous examples.

What is claimed is:
 1. A process for the production of hydrocarbonscomprising the step of contacting a synthesis gas feed comprised ofhydrogen and carbon monoxide over a catalyst comprised of cobalt andrhenium composited on an alumina support wherein rhenium is present inrelatively lesser amounts than the cobalt content of the catalyst, at atemperature, gaseous hourly space velocity and a pressure useful forpromoting a Fischer-Tropsch Synthesis.
 2. The process recited in claim 1wherein the temperature is in the range of about 150° to 325° C.
 3. Theprocess recited in claim 1 wherein the pressure is in the range of about1 to 100 atmospheres.
 4. The process recited in claim 1 wherein thegaseous hourly space velocity in the range of about 0.1 to 50 m³ ofsynthesis gas per kilogram of catalyst per hour is employed.
 5. Theprocess recited in claim 1 wherein the cobalt is present incatalytically active amounts up to about 60 weight percent of thecatalyst.
 6. The process recited in claim 1 wherein the rhenium ispresent in amounts from about 0.5 to 50 weight percent of the cobaltcontent of the catalyst.
 7. The process recited in claim 1 wherein thealumina is gamma alumina.
 8. The process recited in claim 1 wherein thesynthesis gas feed is heated before contacting the catalyst.
 9. Theprocess recited in claim 1 wherein the contacting step takes place in afixed bed reactor.
 10. The process recited in claim 1 wherein thecontacting step takes place in a fluidized bed reactor.
 11. The processrecited in claim 1 wherein the contacting step takes place in anebullating bed reactor.
 12. The process recited in claim 1 wherein thecontacting step takes place in a slurry reactor.
 13. The process recitedin claim 1 wherein the molar ratio of hydrogen to carbon monoxide isbetween 0.5:1 to 3:1.
 14. The process recited in claim 1 wherein themolar ratio of hydrogen to carbon monoxide is between 1.5:1 to 2.5:1.15. The process recited in claim 1 further comprising the step ofcooling the gaseous product of the contacting step to condense a liquidby exposure to progressively lower temperatures.
 16. The process recitedin claim 1 wherein the catalyst is further comprised of a metal oxidepromoter chosen from the group consisting of elements in groups IIIB,IVB and VB of the periodic chart, the lanthanides and the actinides.